1. Field of the Invention
The present invention relates to a method for forming a wafer identifying mark in a manufacturing process of a semiconductor integrated circuit, and particularly to a method for forming an identifying mark on an SOS substrate using sapphire in a manufacturing process of a semiconductor integrated circuit.
2. Description of the Related Art
Since it is possible to reduce a parasitic capacitance between an elemental device such as a transistor and a substrate and decrease a current that leaks from the elemental device such as the transistor to the substrate, it is generally advantageous to fabricate a device satisfactory in high-frequency characteristic, a low power consumption device and the like on a wafer high in insulating property. As the substrate, for example, a silicon-on-insulator (SOI) substrate has been used which has a structure wherein a silicon oxide film (SiO2) is interposed between a silicon (Si) substrate and a silicon layer in a device production area.
Further, as a substrate that uses the above features to the full, there is considered a substrate wherein a silicon layer in a device production area is formed on an insulating layer. As the substrate having such a structure, may be mentioned, a silicon-on-sapphire (SOS) substrate in which a silicon layer in a device production area is formed on a sapphire substrate. The technology of manufacturing a device on the SOS substrate has also started with commercialization.
Comparing a process technology for fabricating a device on an SOI substrate and that for fabricating a device on a bulk silicon substrate currently in vogue, a large difference does not occur therebetween. Thus, an apparatus or the like used in a bulk silicon process can be diverted as it is. It can be said from this point of view that the bulk silicon process and the SOS process can also be carried out simultaneously with the same production lines shared therebetween.
However, a sapphire crystal constituting the SOS substrate is aluminum oxide (Al2O3). When a device is fabricated in a state in which sapphire is being bare, a small amount of aluminum (Al) is adhered to a portion where the sapphire and the apparatus are brought into contact with each other. There is a fear that a phenomenon such as adhesion of aluminum to the apparatus, that aluminum is dissociated from the sapphire during heat treatment at a high temperature and floats in the apparatus, will occur.
It has been known that when such aluminum is captured or taken in a transistor in the course of its manufacture, it leads to degradation of device characteristics. There is a need to make such a contrivance that aluminum is not taken in a device fabricated on the same SOS substrate and devices on other bulk silicon substrate and the SOI substrate both of which share production lines therebetween. Even in the sense of avoidance of such a problem, the SOS substrate is constructed in a state in which a portion other than a silicon layer in a device production area is perfectly covered with a silicon nitride film (Si3N4) or the like, and no sapphire is bared. This method has been used as an out diffusion measure (refer to Japanese Unexamined Patent Publication No. Hei 5(1993)-235006).
When the devices are fabricated on the production lines by use of the SOS substrate, bulk silicon substrate and SOI substrate each having such a construction as described above, some contrivance to identify the background of each substrate, such as a history of process operations, generally, marking of a substrate placed immediately before its introduction into production lines with a wafer identifying mark or the like such as alphameric characters by laser has been carried out (Japanese Unexamined Patent Publication No. Hei 8(1996)-37137, No. 2000-294467 and No. 2001-257139).
However, when the SOS substrate is laser-marked with the wafer identifying mark, the wafer identifying mark is formed in a form that the silicon nitride film or the like for covering up the silicon layer and sapphire in the device production area is burned out. At this time, laser reaches a sapphire layer after the silicon nitride film or the like for covering up the silicon layer and the sapphire in the device production area has been molten and removed.
Therefore, the sapphire is bared at the portion where the wafer identifying mark is formed, thus causing a fear that aluminum will be dissociated during heat treatment at a high temperature in process. In particular, a problem arises in that since a sapphire crystal is damaged by laser, aluminum in the sapphire crystal becomes easy to cause out diffusion.